There have been hitherto suggested a number of attempts to produce superconducting thin films by forming a superconducting layer by forming a film of an oxide superconductor on a substrate, and forming a stabilizing layer for protecting the superconducting layer or the like on the superconducting layer, in order to utilize the superconducting thin films in superconductor cables or superconductor magnets.
Among these, a superconductor wire which is obtained by forming a film on a long tape-shaped substrate by using an RE-based superconductor (RE: rare earth element) represented by the composition formula of REBa2Cu3O7−δas one of oxide superconductors that exhibit superconductivity at the liquid nitrogen temperature (77 K) or higher, can obtain high electric current characteristics, and therefore, such a superconductor is one of the superconducting thin films for which active research and development is currently in progress. Now, it has reached the state that production of a large number of trial products related to electric power equipment and the like using this superconducting thin film is underway.
It is known that the critical current characteristics (Ic characteristics) in such a superconducting thin film are largely dependent on the orientation of crystals that constitute the oxide superconductor. This is because crystals of an oxide superconductor have electrical anisotropy, and it is easy to allow electricity to flow in the a-axis direction and the b-axis direction of the crystals, but it is difficult to allow electricity to flow in the c-axis direction. Therefore, in order to obtain a superconducting thin film having excellent critical current characteristics, the superconducting crystals that form the superconducting thin film need to have satisfactory crystal orientation, and it is necessary to set the direction of the orientation such that the a-axis or the b-axis is in the direction for electricity flow, and the c-axis is in another direction.
In order to obtain such superconducting crystals having satisfactory crystal orientation, it is necessary that the degree of lattice mismatch between the superconducting crystals and an underlying substance be small, and the surface of the underlying substance be compact and smooth. Therefore, with regard to the underlying substance, one having a crystal structure with a small degree of lattice mismatch between the underlying substance and superconducting crystals needs to be selected, and the underlying substance itself also needs to have satisfactory crystal orientation.
In order to obtain superconducting crystals having satisfactory crystal orientation, various configurations of underlying layers and various methods for forming an underlying layer have been hitherto attempted. Representative methods include, for example, a surface oxidation epitaxy (SOE) method, an ion beam assisted deposition (IBAD) method, and a RABiTS (Rolling-Assisted Biaxially Textured Substrate) method. For example, according to an example of the configuration of an underlying layer obtained by using an IBAD method, a Gd2Zr2O7 layer, a MgO layer, a LaMnO3 layer, and a CeO2 layer are disposed in this order on a low-magnetic, non-oriented metal substrate (for example, HASTELLOY (registered trademark, manufactured by Haynes International, Inc.)), which is a nickel-based heat-resistant, corrosion-resistant alloy, and this may be used as an underlying layer. Here, the IBAD method is used when MgO is laminated.
The roles of the respective layers are as follows. The Gd2Zr2O7 layer prevents diffusion of the elements of the metal substrate and suppresses the generation of reaction products between the elements of the metal substrate and the upper layers formed on the Gd2Zr2O7 layer among the laminated underlying layers (for example, the MgO layer, the LaMnO3 layer, and the CeO2 layer). The MgO layer is forcibly oriented by the IBAD method and serves as a base of the orientation of the upper layers. The LaMnO3 layer plays the role of buffering the degree of lattice mismatch between the MgO layer and the CeO2 layer. The CeO2 layer, which is the uppermost layer of the underlying layers, has a small degree of lattice mismatch with the crystals of an RE-based superconductor, and therefore, the CeO2 layer is suitable as an underlying substance for producing an RE-based superconductor having satisfactory crystal orientation.
In view of the configuration and roles of the underlying layers as described above, it is obvious that the surface texture of the substrate affects the orientation of the underlying layers and the RE-based superconductor formed on the substrate. That is, when the surface roughness of the substrate is high, the surface irregularities increase in all of the underlying layers formed on the substrate, and consequently, satisfactory crystal orientation cannot be obtained.
Thus, Patent Document 1 (Japanese Patent Application Laid-Open (JP-A) No. 5-250931) discloses a superconducting thin film that includes a superconducting layer of an oxide having excellent crystal orientation and is capable of reducing the thickness of intermediate layers, by using a smoothened long substrate having a surface roughness Rmax of 0.05 μm an or less.